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Photomixers as tunable terahertz local oscillators [Elektronische Ressource] / vorgelegt von Iván Cámara Mayorga

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145 pages
PhotomixersastunableterahertzlocaloscillatorsDissertationzurErlangung des Doktorgrades (Dr. rer. nat.)derMathematisch-Naturwissenschaftlichen FakultätderRheinischen Friedrich-Wilhelms-Universität Bonnvorgelegt vonIván Cámara MayorgaausMadrid (Spanien)Bonn 2008Angefertigt mit Genehmigung der Mathematisch-NaturwissenschaftlichenFakultät der Rheinischen Friedrich-Wilhelms-Universität Bonn1. Referent: Prof. Dr. Karl Menten2. Referent: Prof. Dr. Karl MaierTag der Promotion: 18 September 2008Diese Dissertation ist auf dem Hochschulschriftenserver der ULB Bonnhttp://hss.ulb.uni-bonn.de/diss online/ elektronisch publiziertDedico este trabajo a mi esposa, Evgenia Muraviova.AbstractThis work reports on the development of the photomixing technology andits immediate application to realize a tunable coherent source in the ter-ahertz (THz) frequency range with an unprecedented bandwidth. An ex-tensiveexperimentalstudyoflow-temperature-growngalliumarsenide(LT-GaAs) and ion-implanted GaAs as photomixing materials is performed inorder to determine the optimal material parameters and fabrication condi-tions.Defect Engineering allows to create photoconducting materials with out-standing properties for THz signal generation. The type and concentra-tionofsemiconductordefectshasacriticalimportanceintheperformanceof the material used for photomixing.
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Photomixersastunableterahertzlocal
oscillators
Dissertation
zur
Erlangung des Doktorgrades (Dr. rer. nat.)
der
Mathematisch-Naturwissenschaftlichen Fakultät
der
Rheinischen Friedrich-Wilhelms-Universität Bonn
vorgelegt von
Iván Cámara Mayorga
aus
Madrid (Spanien)
Bonn 2008Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen
Fakultät der Rheinischen Friedrich-Wilhelms-Universität Bonn
1. Referent: Prof. Dr. Karl Menten
2. Referent: Prof. Dr. Karl Maier
Tag der Promotion: 18 September 2008
Diese Dissertation ist auf dem Hochschulschriftenserver der ULB Bonn
http://hss.ulb.uni-bonn.de/diss online/ elektronisch publiziertDedico este trabajo a mi esposa, Evgenia Muraviova.Abstract
This work reports on the development of the photomixing technology and
its immediate application to realize a tunable coherent source in the ter-
ahertz (THz) frequency range with an unprecedented bandwidth. An ex-
tensiveexperimentalstudyoflow-temperature-growngalliumarsenide(LT-
GaAs) and ion-implanted GaAs as photomixing materials is performed in
order to determine the optimal material parameters and fabrication condi-
tions.
Defect Engineering allows to create photoconducting materials with out-
standing properties for THz signal generation. The type and concentra-
tionofsemiconductordefectshasacriticalimportanceintheperformance
of the material used for photomixing. In LT-GaAs, defects are highly de-
pendent on the arsenic beam equivalent pressure (BEP), growth and an-
neal temperature. Unfortunately, the growth temperature at which an LT-
GaAs sample shows optimal properties lacks very often of fabrication re-
producibility. In contrast to LT-GaAs, the defects created in ion-implanted
GaAs can be tailored by varying the implantation dose and energy. In or-
der to achieve a given concentration of defects, Monte Carlo simulations
wereperformedtodetermineoptimalconditions. Theprecise
control over implantation dose and energy allows to overcome the repro-
ducibility limitations of LT-GaAs.
Photomixers were fabricated patterning Ti/Au interdigitated electrodes by
electronbeamlithographyonthefeedpointofdifferentplanarantennade-
signs (resonant dipoles and broadband logarithmic spirals). Electromag-
netic simulations of the radiating structures are shown. In addition, semi-
conductor simulations were performed, revealing the build-up of space
charge regions next to the electrodes. The problematic of space charge
formation is analyzed and discussed.
Experiments with optimized photomixers demonstrate successfully
pumping of astronomical heterodyne receivers at 450 GHz with a
superconductor-insulator-superconductor(SIS)mixerandat750GHzwith
a hot-electron-bolometer (HEB) mixer. The double sideband (DSB) noise
temperature of the astronomical receiver pumped by a photomixer and by
a solid state local oscillator (both measured at an intermediate frequency
band of 2 to 4 GHz) were identical (T = 170 K).receiver
In addition to the photomixing results, the issue of frequency stabilizationii
of free-running lasers is covered. Experiments were performed using an
optical comb generator as a relative frequency reference. Under the fre-
quency lock condition, the beat signal fulfilled the linewidth requirements
for the photomixing system to be used as a local oscillator for heterodyne
receivers in radio astronomy.Contents
Abstract i
1 Introduction 1
2 Principlesofphotomixing 7
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Basics of photomixing . . . . . . . . . . . . . . . . . . . . . 7
2.3 Carrier injection in recombination- and transit time-limited
photomixers . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4 Photoconduction kinetics . . . . . . . . . . . . . . . . . . . 10
2.5 Modeling of small-area recombination lifetime-limited pho-
tomixers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3 Materialsforterahertzphotomixing 15
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2 Optical absorption in gallium arsenide . . . . . . . . . . . . 15
3.3 Defects in gallium arsenide . . . . . . . . . . . . . . . . . . 16
3.4 Measurement of subpicosecond carrier dynamics . . . . . . 19
3.5 Low-temperature-grown GaAs: Fabrication and control of
defects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.6 Ion implanted gallium-arsenide . . . . . . . . . . . . . . . . 23
4 MeasurementsetupforTHzphotomixing 27
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.2 Optical system . . . . . . . . . . . . . . . . . . . . . . . . . 27
4.3 The Photomixing device . . . . . . . . . . . . . . . . . . . . 33
4.3.1 The MSM structure . . . . . . . . . . . . . . . . . . . 34iv CONTENTS
Capacity of the interdigitated structure . . . . . . . . 34
Build-up of space-charge regions . . . . . . . . . . . 35
Simulations of the photomixing device . . . . . . . . 40
4.3.2 Quasi-optics . . . . . . . . . . . . . . . . . . . . . . 48
4.3.3 Antenna designs . . . . . . . . . . . . . . . . . . . . 51
Resonant designs . . . . . . . . . . . . . . . . . . . 54
Broadband designs . . . . . . . . . . . . . . . . . . 56
5 MeasurementswithLT-GaAsphotomixers 61
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.2 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.2.1 The effect of bias voltage . . . . . . . . . . . . . . . 62
5.2.2 The effect of laser power . . . . . . . . . . . . . . . 65
5.2.3 Cryogenic operation . . . . . . . . . . . . . . . . . . 68
6 Measurementswithion-implantedGaAsphotomixers 73
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.3 Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
6.4 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . 76
7 TerahertzphotonicmixersaslocaloscillatorsforSISandHEB
receivers. 81
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
7.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
7.3 Mixing experiment with an SIS receiver at 450 GHz . . . . . 82
7.4 Mixing experiment with a HEB at 750 GHz . . . . . . . . . . 85
8 Conclusionsandfuturework 91CONTENTS v
A Relativefrequencystabilizationoffree-runninglasers. 93
A.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
A.2 Motivationforthefrequencystabilizationoffree-runninglasers 93
A.3 The optical comb generator . . . . . . . . . . . . . . . . . . 95
A.4 The electro-optic modulator . . . . . . . . . . . . . . . . . . 98
A.5 Measurements with the optical comb . . . . . . . . . . . . . 99
A.6 Relative frequency stabilization with a comb generator . . . 105
A.7 Conclusions and future work . . . . . . . . . . . . . . . . . 106
Bibliography 107
Acknowledgments 115
PublicationsandConferences 117
CurriculumVitae 131

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